The invention relates to a differential cling forage wrapping film, a wrapped bale, a method for manufacturing a differential cling film, and a method for wrapping forage using a differential cling film.
Packaging materials are available for agricultural products such as hay and other forage products for the production of silage. Forage wrapping films are generally intended to wrap forage products and hold or retain the moisture within the product. Forage wrapping films are different from conventional pallet wrapping films which are used to wrap non-agricultural, unitized loads. Pallet wrapping films generally exhibit much higher stretch and much lower cling than forage wrapping films. In addition, they generally are not required to withstand the conditions associated with being exposed to weather in a field for up to about 12 months.
Prior forage wrapping films can be categorized as either blown films or cast films. Exemplary blown forage wrapping films are available under the names BaleGard(copyright) from Presto Products Company of Appleton, Wis., Sunfilm(copyright) from AEP, and MaxTech(trademark) from Tenneco, Inc. Blown forage wrapping films generally exhibit a large differential cling value between the outer surfaces of the film. For example, one surface of the film generally exhibits a very low cling value and the opposed surface of the film generally exhibits a very high cling value. It is understood that this generally results from the blowing process where a migratory cling additive migrates to the outer surface of the blown film.
A prior cast film is also available under the name BaleGard(copyright) from Presto Products Company of Appleton, Wis. This prior cast forage wrapping film can be categorized as a one side cling film. That is, one side of the film exhibits a high degree of cling, and the other side of the film exhibits essentially no cling.
For forage wrapping films, it is generally desirable to provide one side of the film with a high cling value. This property allows the film to stick to itself after it has been used to wrap a bale. The high cling side of the film faces inside when it wraps a bale. The reason for this is that if the high cling side were on the outside of the bale, the film would have a tendency to stick to other bales or to machinery. Forage wrapping films are generally provided in a roll form for use in wrapping bales. The high cling side is on the outside of the film when it is provided in roll form so that it does not stick to the machinery when it is used for wrapping bales.
Various methods have been disclosed to provide films having differential cling properties whereby one side of a wrap film exhibits strong cling properties while the other side of the film exhibits little or no cling properties. Three general techniques have been relied upon for providing films having differential cling properties. A first technique for providing a differential cling film include providing a laminate of at least two different polymer layers where one side of the film has the desired cling properties. This technique often involves the use of a tackifier and is described by U.S. Pat. No. 5,175,049 to Huff et al. Another technique for providing a differential cling film includes altering the cling properties of one side of the film by surface treatment (e.g., chemical, thermal, and/or electrical, such as corona discharge). A third technique involves the addition of tackifiers which are blended into the principal film polymer and the blend is then extruded and cooled as a film in such a way as to cause the tackifiers to exhibit differential cling properties when applied as wrap-packaging material.
A differential cling film is provided by the present invention. The differential cling film of the invention includes at least three layers. The first layer can be referred to as the core layer, the second layer can be referred to as the high cling layer, and the third layer can be referred to as the low cling layer. The high cling layer exhibits a same side cling value which is greater than the same side cling value of the low cling layer. Preferably, the high cling layer exhibits a same side cling value of between about 300 grams at 0% stretch and 550 grams at 100% stretch, and said low cling layer exhibits a same side cling value of between about 40 grams at 0% stretch and 125 grams at 100% stretch. The differential cling film preferably exhibits an opposite cling value in the range of between about 175 grams at 0% stretch and about 375 grams at 100% stretch. The same side and opposite side cling values are determined according to ASTM D5458-95.
In order to function properly as a forage wrapping film, the differential cling film of the invention should be sufficiently tough to resist puncturing and tearing. A preferred differential cling film exhibits a dart impact value according to ASTM D1709-91 of greater than about 150 grams, a MD tear value according to ASTM D1922-94 of greater than about 150 grams, and a TD tear value according to ASTM D1922-94 of between about 500 grams and about 800 grams.
The core layer of the differential cling film is primarily responsible for providing the desired toughness. On a mass basis, the core layer generally constitutes between about 70 percent and about 90 percent of the differential cling film. Preferably, the core layer includes polyethylene polymer. In order to increase toughness, the polyethylene polymer can be blended with an impact modifier. Preferred impact modifiers include polyethylene plastomer and ultra low density polyethylene. A preferred polyethylene plastomer is a metallocene catalyzed linear low density polyethylene.
The high cling layer preferably includes a blend of ultra low density polyethylene and non-liquid tackifier. The low cling layer preferably includes a blend of polyolefin and non-liquid tackifier. The non-liquid tackifier is preferably one which is solid at room temperature and atmospheric pressure, and is one which will not migrate through the film to other layers or to the surface of the film in an amount which will result in significant rubbing off of the tackifer onto the hands of the user. A preferred non-liquid tackifier is amorphous polyolefin, such as amorphous polypropylene.
A wrapped bale is provided by the present invention. The wrapped bale includes a forage product wrapped by a differential cling film. In general, the forage product is wrapped and left outdoors for several weeks to about one year. The purpose of the differential cling film is to retain much of the moisture within the forage product as the forage product converts to silage.
A method for producing a differential cling film is provided by the present invention. The method includes a step of providing at least three polymer melts and casting the three polymer melts to provide a multilayer film exhibiting differential cling properties. In a preferred embodiment, the first polymer melt includes a polyethylene polymer, the second polymer melt includes a blend of ultra low density polyethylene and non-liquid tackifier, and third polymer melt includes a blend of polyolefin and non-liquid tackifier.
A method for wrapping forage is provided by the present invention. The method includes steps of providing forage product in bale form, providing a differential cling film, and wrapping the bale with the differential cling film.